In an electrical apparatus applied with a high voltage, such as a motor or the like used at a high voltage, for example, the high voltage is applied to an insulated wire forming the electrical apparatus, and for this reason, a partial discharge (corona discharge) is easily generated at a surface of an insulating layer of the insulated wire. An insulation breakdown occurs at an early stage if a local temperature rise, generation of ozone, generation of ions, or the like are caused by the generation of the corona discharge, and as a result, a service life of the insulated wire and thus the electric apparatus is shortened. Accordingly, the insulated wire used in the electric apparatus applied with the high voltage is required to have an improved corona breakdown voltage, in addition to good insulating properties, mechanical strength, or the like.
As a means of increasing the corona breakdown voltage, it is effective to lower a dielectric constant of the insulating layer. In order to lower the dielectric constant of the insulating layer, an insulated wire, which forms a heat-curing layer (insulating layer) by an insulating varnish including a film-forming resin and a pyrolytic resin decomposable at a temperature lower than a baking temperature of the film-forming resin, has been proposed (refer to Japanese Laid-Open Patent Publication No. 2012-224714). According to this insulated wire, pores are formed within the heat-curing layer by utilizing pores that are formed by thermally decomposed portions of the pyrolytic resin when baking the film-forming resin, and the dielectric constant of the insulating coating can be lowered by the forming of the pores.